What is your feeding schedule like? You said he would eat large rats twice a month. I personally feed sm/md rats every 7 days and this seems to work well. Occasionally I'll "retire" my breeder rats to my BIG females, but thats once in a while. Most of my snakes eat pre-killed rats but some eat live and have never converted over. I had one occasion in which a larger rat fought back and it shut one of my snakes down from feeding for about 5 weeks. I offered it smaller prey and she's back on with a vengance. I also have hides which seemed to help with some of my slower feeders. They picked up right away and eat much more consistently. Of cousre, check your heat, husbandry, how often he poops. He may be full and not hungry. Some snakes go off feed due to breeding season, and some just do it to annoy us ownersJust keep offering weekly not every day as that can also stress an animal that may not want to eat.